1. Field of the Invention
The present invention relates to an image forming apparatus utilizing electro-photography such as a copying machine, a printer, a facsimile machine or a machine having a combination thereof. More particularly the present invention relates to an image forming apparatus capable of maintaining a stable image for a long time by preventing fluctuation of the image depth caused by developing history or successive printing in a hybrid developing system that develops a latent image on an electrostatic latent image bearing body (a photosensitive body) by flying the charged toner to jump thereto from a developing roll which holds charged toner selectively using a two component developer material which charges non-magnetic toner by means of a magnetic carrier.
2. Description of the Related Art
Heretofore, a non-contact, one component developing method which develops a latent image on an electrostatic latent image bearing body (a photosensitive body) by causing the charged toner to jump thereto from a developing roll which holds charged toner selectively using a two component developer material which charges non-magnetic toner by means of magnetic carrier has been investigated. Recently, however, a similar method has been investigated as a method capable of forming the image rapidly, particularly as a single-roll-pile-up-color method by which a plurality of color images are piled up on an electrostatic latent image bearing body (a photosensitive body). The method attracts attention as a technique for high quality color images because piling the toners precisely up on the electrostatic latent image bearing body can form a color image of minimum color drift.
Recently, a so called tandem method has attracted attention because of its high printing rate, by which color layers are piled on a recording sheet using a plurality of photosensitive bodies, each body corresponding to each color of the multi-colors, by forming color images thereon synchronized with the transfer of the sheet. Although the method has an advantage in its high printing rate, the apparatus tends to be large-sized because each electro-photographic process member of each color needs to be disposed in rows. To cope with this disadvantage, a compact tandem type image forming apparatus having a small-sized image forming unit wherein the distances among the photosensitive bodies in the array are shortened has been proposed.
In a compact tandem type image forming apparatus that is constructed in this way, it is favorable that a developing unit is vertical in order to minimize the latitudinal size of the image forming unit. In other words, it is preferable to dispose the developing unit above the photosensitive body in light of layout. Nevertheless, problems are that, in the case of the vertical arrangement of the developing unit in the conventional two component developer material method, assembling of the apparatus to a compact body is limited and at the same time, it is inevitable that carrier attaches the photosensitive body and toner scatters owing to lowering of charge on the toner because supplying developer material from a developer stirring part to a developing member adjacent to the photosensitive body, i.e. a reflux of developer material, becomes complicated.
Though one component method without carrier is proposed as another method, torque fluctuation of a rotational photosensitive body by contacting a developing roll to a photosensitive body results in color drift which is a drawback in a tandem method. In a method in which a photosensitive body is not touched, toner is charged with a charge roll and an elastic restricting blade defines a layer thickness on the developing roll. Therefore, toner additives adhere to the charge roll to lower charge capability or toner adheres to the restricting blade to form an uneven layer. These result in image defects.
As a measure to solve these problems, a hybrid developing method has been proposed wherein toner is charged using an aforementioned two-component developer material and the toner alone is flown to a photosensitive body without subsequent contact. An example of such prior art is disclosed in U.S. Pat. No. 3,866,574, in which the inventor proposed that a thin layer of non-magnetic toner was formed on a donor roll (developing roll) disposed so as not to come in contact with an electrostatic latent image bearing body (a photosensitive body) and then the toner was caused to jump to a latent image on the electrostatic latent image bearing body (the photosensitive body) by an alternating current electric field. Further, U.S. Pat. No. 3,929,098 discloses a hybrid developing unit wherein a toner layer is formed on a donor roll by advancing developer material onto a donor roll to transfer thereon.
Toshiba Corporation reported a theoretical aspect of formation of toner layer on a developing roll using two component developer material in the Journal of the Imaging Society of Japan, Vol. 19, No. 2, 1981 and the art is filed and disclosed on Japanese laid-open patent, JP1984-121077.
Nevertheless, these arts need strong alternating current electric field in order to overcome the difficulty in separating toner on the roll when the electric charge of the toner becomes high, although formation of a thin layer on the donor roll is possible by adopting the two component developer material method. The strong alternating current electric field disorders the toner layer on the electrostatic latent image bearing body (the photosensitive body) so that difficulty arises in color pile up. Considering the overcoming of this drawback, a so-called powder cloud developing method is proposed in Japanese laid-open patent, JP1992-113474, in which an auxiliary electrode is provided to apply a weak alternating current electric field so as not to disorder the developed toner image. The aforementioned prior arts need complicated control of electrification such as application of high surface potential and strong developing electric field on the photosensitive body. Hence, when both toner consuming region and toner non-consuming region are generated, adhering states of toner and toner potentials disperse on the roll so that a hysteresis tends to occur, that is a phenomenon in which a ghost image of a part of a previous developing image appears on successive developing images as shown in FIG. 4. In FIG. 4, 13 is a solid image consisting of a rectangular solid black and 16 is a successive half tone image broader than the solid image. In case a toner consuming region and a toner non-consuming region are generated on the developing roll, a ghost image 14A shown in FIG. 4(b) appears when printing the half tone image 16 after the solid image 13.
To avoid this defect, Japanese laid-open patent publication, JP1999-231652 discloses a member for scraping residual developing toner on the developing roll and an apparatus for recovering the scraped toner. Further, as a method for recovering completely toner on the developing roll, Japanese laid-open patent publication, JP2000-81788 discloses a recovery roll for sole use in recovery. Further, a method for stabilizing charge on toner by recovering toner on the developing roll to a magnetic roll by means of potential difference to stabilize the charge of toner utilizing a passing moment between a copying process and another successive copying process or a moment between the passing of successive sheets.
Japanese laid-open patent publication JP1995-12898 discloses a measure for a hysteresis when using a magnetic brush, which includes a proposal to recover and supply toner on a developing roll by setting the width of the region of half value of magnetic flux density on the magnetic roll to be broad. A control method for a tandem type developing unit disclosed in Japanese laid-open patent publication JP1988-249164 includes a method for preventing deterioration of developer material by interrupting the operation of developing units in image forming parts except for the image forming part which is conducting a transfer process. Japanese laid-open patent publication JP1994-67546 discloses the art of a high frequency voltage application where a high frequency electric field to supply toner to a developing roll from a magnetic roll or a high frequency electric field to exfoliate toner from the developing roll to deliver thereof to the magnetic roll is alternately formed by applying alternately a high frequency electric voltage between the developing roll and the magnetic roll, and the art of recovering toner on a developing roll where toner on the developing roll is recovered by exfoliating electrically with an exfoliating member provided adjacent to a magnetic roll and followed by returning the recovered toner to the magnetic roll.
The powder cloud developing method disclosed in the aforementioned Japanese laid-open patent publication JP1991-113474 has not been, however, paid general attention because a wire for the auxiliary electrode is apt to get dirty and image deterioration occurs by vibration. The apparatuses disclosed in Japanese laid-open patent publications JP1999-231652, JP1995-72733, JP 1995-92804, JP1995-128983, JP1984-121077 and JP2000-81788 have such drawbacks as prerequisites for a member for scraping toner or a recovery roll, as a potential cause for deteriorating durability performance of toner owing to increasing stress of toner by applying special recovery bias voltage and as a loss of speed because of the necessary time for formation of a layer on the developing roll at the successive developing process. These prior arts also have a cause of scatter of toner or fogging owing to insufficient electric charge by broadening a charge distribution of replenished toner or recovered supplied toner, as charge characteristic of toner on the developing roll varies to a large extent by deterioration of durability performance of toner when it is used for a long time. Moreover, as it is troublesome to change deteriorated carrier, the arts have actually been of no practical use.
Apparatuses disclosed in Japanese laid-open patent publication JP1988-249164 and JP1994-67546 necessitate such an apparatus or a control as interrupts operation of developing units other than the developing unit of an image forming part which is under transfer process or as shifts high tension high frequency voltage applied between the developing roll and the magnetic roll. At the same time, the developing unit is constructed so that the toner roll, the magnetic roll and the stirring member are arrayed laterally, which results in difficulty in achieving a compact design.
Furthermore, in these conventional arts, the amount of toner in the toner layer on the developing roll varies when the second developing cycle is brought into operation after the first developing cycle together with a phenomenon which brings about a decline of image depth in the second developing cycle or the later cycle after a deep color developing owing to an insufficient amount of toner. In order to avoid these phenomena, such means may be devised as ensure a sufficient amount of toner by idling the developing roll until the forthcoming developing period or ensure an idle period between successive developing periods. These means do not solve the aforementioned problems as the charge of the toner declines on account of an increase in the load on the developer material if the developing roll runs fast or if the machine is left idle for a considerable length of time.
These conventional arts have a drawback of a possible hysteresis, which means the appearance of a ghost image as shown in FIG. 4(b), to overcome.
In one of the improvements on these conventional arts, the occurrence of the hysteresis is avoided as recovering residual toner on the developing roll to the magnetic roll by applying a potential difference between the developing roll and the magnetic roll at an image-non-forming period, which is the reverse to that at an image-forming period. It is true that to apply a potential difference between both rolls is effective to recover the residual toner on the developing roll to the magnetic roll and the greater the potential difference the easier the residual toner can be recovered.
However, the developing roll needs to rotate in order to recover a lot of layers of toner particles with regard to the mean diameter of toner particles by sticking the residual toner to the carrier of the magnetic brush formed on the magnetic roll. Thus, the residual toner is recovered by rotation of both rolls.
If that is the case, contacting occasions of the magnetic brush to the developing roll can increase by setting a circumferential rotational speed of the magnetic roll greater than that of the developing roll. In addition, the adhesion power of the toner on the developing roll thereto can be diminished as a shear stress by the magnetic brush affecting to the residual toner on the developing roll is strengthened.
As a result, a strongly magnetic carrier can effectively recover the residual toner at an equal potential without applying a potential difference between the developing roll and the magnetic roll. The inventors paid attention to this fact.
Nevertheless, as shown in FIG. 17(b), when an image is formed on a recording sheet 17 shown in (a) where a toner layer is formed in the height direction on a developing roll 32 having a width of H1 and a magnetic brush is formed in the height direction on a magnetic roll 31 having the same width of H1, and both rolls are disposed at the same position in developing vessels 12a and 12b, ghost images 14B, 14C are formed on both edges.
Referring to FIG. 16(a) and 16(b), it is understood that though a residual toner after developing 15a is recovered by a carrier 24A1, a residual toner after developing 15b is recovered by a carrier 24A2 and a residual toner after developing 15c is recovered by a carrier 24A3 in case the developing roll and the magnetic roll are disposed at the same height and at the same size as shown in FIG. 16(a), in case the width in the direction of height of the developing roll 2 is higher than that of the magnetic roll 1, the toner 15c is recovered by the carrier 24B while the toner 15a and 15b are not recovered because magnetic brushes are not formed at the position higher than that of carrier 24B as shown in 16(b).
Thus, the residual toners after developing 15a, 15b and 15c can be recovered by carriers 24C2, 24C3 and 24C4 of the corresponding magnetic brushes by means of forming the width in the direction of a sleeve axis H3 of the magnetic brush on the magnetic roll comparatively longer than the width in the axial direction H1 of the toner layer on the developing roll.
Toners 15d, 15e and 15f of carriers on the magnetic brush jump to and are caught on a lateral side of the developing roll by a potential difference applied between the developing roll and the magnetic roll when forming an image. When not forming an image, the residual toners 15a, 15b and 15c are recovered by the magnetic brush while the toners 15d, 15c and 15e do not touch the magnetic brush to remain unrecoverable, which results in contaminating the inner of the apparatus by scattering.
However, the apparatuses disclosed in Japanese laid-open patent publication JP1999-231652 and JP1996-128983 have a cause to deteriorate durability of toner owing to increasing stress to toner by a toner scraper or application of special bias for recovering toner. These prior arts also have a cause of scatter of toner or fogging owing to insufficient electric charge by broadening a charge distribution of replenished toner or recovered supplied toner, as charge characteristic of toner on the developing roll varies to large extent by deterioration of durability performance of toner in case of using for a long time. Moreover, it is troublesome to change deteriorated carrier so that the arts have actually been of no practical use.
Hybrid type developing units disclosed in Japanese laid-open patent publication JP1994-67546, JP1995-72733 and JP1995-792804 include a powder cloud developing method having an electrode provided between a donor roll and a latent image bearing body (photosensitive body). The method has a drawback that a wire for the electrode is apt to get dirty and has a possibility of image deterioration by vibration. In a developing unit disclosed in Japanese laid-open patent publication JP1995-72733 or JP1995-92804, a toner layer on a developing roll is formed by applying a definite voltage from a magnetic brush so that an electric charge of toner on the developing roll becomes higher in comparison with that of toner in the magnetic brush (in the two component developer material) as repeatedly receiving electric field or friction from the magnetic brush in case of such as a successional printing. In the developing unit, toner on the developing roll is recovered by scraping by the magnetic brush practically without replacing with the toner in the magnetic brush so that the recovery process can be carried out in a short time. Contrary to that, a charge distribution of the toner in two component developer material becomes broad, even generating reversely charged toner for the toner on the developing roll having different charge mixes instantaneously with the toner in the magnetic brush, which brings about disadvantages such as increasing scattering of toner from the magnetic brush or image defect.
An apparatus disclosed in Japanese laid-open patent publication JP2000-250294 or JP2000-250295 includes a developing unit which carries out hybrid development by generating a toner cloud with an electrode embedded in a donor roll (developing roll). It has such complicated construction that brushes for applying charge to electrodes are provided in two places where toner is filed to the donor roll from developing place and the magnetic roll. This pushes up the manufacturing cost for the apparatus.